The present invention relates generally to materials and methods used for depositing thermal barrier coatings on gas turbine blades/vanes using Electron Beam Physical Vapor Deposition (EBPVD) combined with Ion Beam Assisted Deposition (IBAD).
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. Ceramic thermal barrier coatings (TBCs) have been used since the 1970's to protect gas turbine blades and vanes (typically made of iron, nickel, or cobalt-based superalloys) from oxidation and corrosion; and to minimize their service temperatures by providing thermal insulation. Zirconia (ZrO2) that is partially or fully-stabilized by yttria (Y2O3), magnesia (MgO) or other oxides is widely employed as the TBC. TBCs commonly use YSZ (Yttria Stabilized Zirconia), which can contain 2-17% yttrium, and more usually 6-8% yttrium. Zirconia-based TBCs (e.g., YSZ) produced by electron beam physical vapor deposition (EBPVD) have superior adhesion and enhanced thermal strain compliance due to their columnar grain morphology. However, they possess higher thermal conductivities (e.g., 2 W/m-K) compared to plasma-sprayed YSZ (e.g., 1 W/m-K). The plasma sprayed coatings have a lamellar, splat-like, layered microstructure that increases thermal resistance. The higher thermal conductivity of EBPVD coatings is believed to result from the deposition of relatively high-density columnar grains oriented perpendicular to the substrate, with little internal microporosity or other defects to resist the flow of heat. A lower thermal conductivity is desirable in order to increase component lifetimes by allowing a thinner, lighter ceramic coating to be used; and to achieve higher turbine efficiencies due to the possible increase in gas temperature and due to a reduction in the cooling-power requirements for cooling the turbine blade through internal cooling passages.
FIG. 1 shows a simplified cross-section view of a typical structure of a gas turbine blade/vane coated with a TBC. The superalloy substrate 12 (e.g., a nickel-based superalloy, such as Inconel 718), is directly coated with a oxidation-resistant layer called a “bondcoat” 14, which is typically made of 1) a metallic alloy of MCrAlY, where M is iron, cobalt and/or nickel; 2) a binary NiAl alloy, or 3) an oxidation-resistant intermetallic compound, e.g., nickel-aluminide (Ni2Al3) or platinum aluminide (PtAl). The most common bondcoat used on nickel-based superalloys is diffusion nickel-aluminide, typically 25-125 microns thick, and more commonly around 40-50 microns thick. It is conventionally made by diffusing a layer of pure aluminum (e.g., 30-50 microns thick) into the superalloy substrate at 800-900° C. for 2-3 hours, whereupon the diffused aluminum reacts with the underlying nickel to form a well-adhered, nickel-aluminide bondcoat 14.
On top of the bondcoat 14 is a thin adhesion layer 16, typically comprising pure alumina. The adhesion layer 16 is typically made by the following process. A few microns of pure aluminum are usually left over on the surface of the nickel-aluminide bondcoat following the 2-3 hour aluminiding step mentioned above. After performing an optional grit-blasting step to clean and roughen the surface, the aluminum is then controllably oxidized into a few microns of pure alumina (Al2O3) by heating the substrate to an elevated temperature (e.g., 800-1000° C.) in an oxygen-containing atmosphere for 2-4 hours. This layer of thermally grown oxide (TGO) is about 0.1-2.0 microns thick, and preferably about 1 micron thick. Excessively thick TGO layers (e.g., 10 microns) can cause premature TBC failures. It is difficult to achieve precise control of the thickness of the TGO adhesion layer 16 due to many variables, including: the local oxygen concentration adjacent the part in the vacuum chamber, impurities (such as water vapor), non-uniform pre-heat temperature profiles across the part, the aluminum activity in the bond coat, surface impurities, exposure time, and specific temperature history (e.g., a longer heat-up time is needed for heavier parts).
Next, a zirconia-based ceramic TBC layer 18 is deposited to a thickness of about 30-300 microns directly on top of the adhesion layer 16, usually by an EB-PVD process. The alumina adhesion layer 16 provides good chemical bonding and adhesion of the EBPVD YSZ layer 18 to the underlying aluminide bondcoat. The YSZ layer deposited by EBPVD has dense, columnar grains oriented substantially perpendicular to the surface of the substrate, extending outwards from the bondcoat. Before starting EBPVD, the substrate 12 is typically pre-heated to about 900° C. During coating the substrate gradually increases to about 1100° C. due to radiant heating from the molten YSZ ingot (at 2300 C), and from e-beam electrons reflected from internal surfaces. Oxygen can be injected during EBPVD deposition to help insure fully stoichiometric ceramic coatings. Unfortunately, a thermally grown alumina (TGO) layer can continue to grow thicker during a conventional EBPVD process, due to the high substrate temperature (900-1100° C.). This can result in an excessively thick or non-uniform TGO adhesion layer 16.
In addition to needing accurate control over the thickness, composition, and uniformity of the alumina adhesion layer 16, it is useful to have the best crystallographic phase of the alumina. The most-desirable polymorph of alumina is alpha-phase alumina (α-Al2O3), because it has the highest thermal stability and a low oxygen diffusivity. Typically, an alpha-phase alumina layer 16 is formed by post-coating annealing at high temperatures of the TGO alumina layer. This requires annealing at 1200 C for 9 hours to convert the amorphous alumina into a “mature” alpha-phase alumina. By “mature”, we mean that at least 90% of the alumina comprises the alpha-phase, α-Al2O3. The literature reports that post-coating annealing at 1000° C. for 9 hours was unable to convert the amorphous alumina; while annealing at 1100° C. for 1 hour only partially produced an alpha-phase.
EBPVD deposition of alumina has also been studied. According to the literature, alumina coatings deposited via EBPVD on substrates heated to as high as 980° C. still had an amorphous structure. Only when doing EBPVD at substrate temperatures greater than 1100° C. did a “mature” alpha-phase form.
It has also been reported that grit-blasting the aluminide bondcoat 14 prior to depositing EBPVD alumina helps to nucleate the desired alpha-phase. Alternatively, it has been reported that embedding (seeding) small particles of alpha-phase alumina into the bondcoat prior to depositing alumina has also been reported to help nucleate the desired alpha-phase.
It is also desirable to reduce the oxygen partial pressure during EBPVD deposition as much as possible for a number of reasons, including extending the to lifetime of hot cathode electron emitters (e.g., in the ion source, e-beam gun, or other exposed hot filaments).
Reducing the total processing time will greatly reduce costs by increasing the throughput rates. Up to 12 hours of processing time could be saved by eliminating the need to thermally-grow the alumina adhesion layer (i.e., alumina “scale), and by eliminating the need to perform high-temperature annealing to convert the alumina scale in the desirable alpha-phase. These savings can be achieved by combining Ion Beam Assisted Deposition (IBAD) with EBPVD. Against this background, the present invention was developed.